FIG. 1 is a schematic view of a hydrocarbon reservoir. In essence, a hydrocarbon reservoir consists of layers of the earth's interior in which hydrocarbons have accumulated. Only one hydrocarbon-bearing layer 2 is shown in FIG. 1, for convenience of description. In FIG. 1 the structure of the earth overlying the hydrocarbon-bearing layer 2 is represented as a single layer 1, but in practice it will consist of many layers having different geological compositions. Similarly, the structure of the earth below the hydrocarbon-bearing layer 2 will in practice consist of many layers but is represented in FIG. 1 as a single layer 3.
Microseismic activity, or “microseismicity”, in hydrocarbon reservoirs can be induced as a result of extraction of hydrocarbons from the reservoir, which has the effect of altering the weight distribution of the hydrocarbon-bearing layer 2 and of the overlying layer 1. Microseismicity may also be induced by hydraulic fracturing operations. When microseismic activity occurs, seismic waves are generated at a point within the earth's interior. The seismic waves propagate through the earth, and may be detected by seismic receivers located within the earth. Microseismic activity induced by hydrocarbon extraction is weak and cannot generally be detected by seismic receivers at the earth's surface, although earthquake-induced microseismic activity can generally be detected by receivers at the earth's surface.
In principle, microseismic activity can occur anywhere in the reservoir layer 2 of FIG. 1, in the overlying layer 1 or in the surrounding rock. In practice, however, the earth's structure contains geological fractures or other faults, and microseismic activity preferentially occurs along or in the vicinity of such faults or fractures. In FIG. 1, a fault 4 is schematically depicted in the hydrocarbon bearing layer 2. It is assumed that the earth's interior to the right of the fault 4 is tending to move downwards whereas the earth's interior to the left of the fault 4 is tending to move upwards as indicated by the arrows A,B in FIG. 1. If a portion of the fault at one location 5 slips, microseismic activity occurs at that location 5. The slippage will relax tension at the location 5 on the fault where slippage occurs, but will induce tension at neighbouring locations along the fault and this increased tension may give rise to slippage, and consequent microseismic activity, at a nearby location 6 on the same fault 4. The time delay between microseismic activity at one location 5 and microseismic activity at the nearby location 6 may be of the order of seconds or minutes, or it may be of the order of days or even weeks.
The seismic energy produced by the microseismic activity occurring at locations 5,6 will be detected by seismic receivers 7,8,9 disposed in a borehole 10. The seismic data acquired at the receivers 7,8,9 therefore contain events arising from microseismic activity—or “microseismic events”—in addition to microseismic events arising from other faults in the layers 1,2.
Where microseismic activity occurs along a geological fault or fracture, microseismic activity is, as explained above, often found to occur at two nearby locations, such as the locations 5,6 in FIG. 1. Since the two locations are near to one another, the focusing effects of the overlying layers on seismic waves emitted at one location 5 will be similar to the focusing effects of the overlying layers on seismic waves emitted at the nearby location 6. Furthermore, since the microseismic activity at each location 5,6 arises from slippage of the fault 4, microseismic activity at one location 5 has the same source mechanism as microseismic activity at the nearby location 6. Thus, microseismic activity occurring at location 5 and microseismic activity occurring at the nearby location 6 would produce similar recordings at a seismic receiver, since the two locations are close to one another and have similar focusing mechanisms to one another, and since the microseismic activity has the same source mechanism at each location. Data acquired at a receiver will therefore contain an event corresponding to microseismic activity at location 5 and an event corresponding to microseismic activity at location 6. The two events will have a similar form, and will be separated by a time delay of, typically, from seconds to days or even weeks. Such pairs of events in acquired seismic data are known as multiple acoustic emissions or “doublets”. The effect of the relative positions of the receiver and the event locations 5,6 on the time delay between the two events acquired at a receiver is small, typically of the order of
10−1 to 10−2 seconds, so the time delay is determined primarily by the time delay between microseismic activity at one location and microseismic activity at the other location. (The effect of the relative locations of the receiver and the events on the time delay between the two events arises primarily from a change in the azimuth of the receiver with respect to the doublet orientation.)
It is desirable to identify doublets in seismic data acquired at seismic receivers in a hydrocarbon reservoir. As is known, once a doublet has been identified it is possible to determine the relative location of the microseismic activity giving rise to each event (the location of the microseismic activity giving rise to an event will be referred to as the “location of an event”, for convenience). The relative locations of the two events of a doublet—that is, the location of one event of the doublet relative to the location of the other event—may be determined more precisely then their absolute locations. The events and their accurate relative locations can be used for fault delineation, permeability estimation, tracking of flood fronts, stress transfer, etc.